A general photocoupler has a configuration in which two lead frames are disposed facing each other with a gap therebetween, a light emitting device (for example, a light emitting diode) is mounted and bonded to a lead frame on an input side, a light receiving device (for example, a light receiving IC) is mounted and bonded to a lead frame on an output side, and the light emitting device and the light receiving device are disposed facing each other and with a gap therebetween (for example, refer to FIG. 5 of Patent Document 2). In this type of photocoupler, since parasitic capacitance (floating capacitance) exists equivalently between input and output (between the light emitting device and the light receiving device), when voltage that changes suddenly due to a noise signal or a high speed switching operation is added to the parasitic capacitance, a current (displacement current) flows from the input side (the light emitting device and lead frame) to the light receiving device even when the light emitting device is not on, and an output malfunction may occur.
With regard to this malfunction, there is what is called CMR (Common Mode Rejection) as an index representing how hard it is to have a malfunction. The CMR relates to through rate of a maximum common mode voltage VCM that can be maintained with an output state of the photocoupler in a correct state, and is represented as a voltage change value per 1 μs (refer to Non-Patent Document 1).
In the general photocoupler as described above, a displacement current that flows, through parasitic capacitance between input and output, due to the voltage suddenly changing by a noise signal or a high speed switching operation, can be represented as in Formula 1.
                                          i            d                    =                    ⁢                                                    ∫                S                            ⁢                                                                                          ∂                      D                                                              ∂                      t                                                        ·                  n                                ⁢                                  ⅆ                  S                                                      =                        ⁢                                                                                ⅆ                                                                                                                      ⅆ                    t                                                  ⁢                                                      ∫                    S                                    ⁢                                                            D                      ·                      n                                        ⁢                                          ⅆ                      S                                                                                  =                            ⁢                                                                                          ⅆ                                                                                                                                  ⅆ                      t                                                        ⁢                                                            ∫                      V                                        ⁢                                          divD                      ⁢                                              ⅆ                        V                                                                                            =                                ⁢                                                                                                    ⅆ                                                                                                                                              ⅆ                        t                                                              ⁢                                          (                                              ∫                                                  ρ                          ⁢                                                      ⅆ                            V                                                                                              )                                                        =                                    ⁢                                                                                                              ⅆ                                                                                                                                                          ⅆ                          t                                                                    ⁢                                              (                        Q                        )                                                              =                                        ⁢                                                                                                                        ⅆ                                                                                                                                                                      ⅆ                            t                                                                          ⁢                                                  (                          CV                          )                                                                    =                                            ⁢                                                                        C                          ⁢                                                                                    ⅆ                              V                                                                                      ⅆ                              t                                                                                                      =                                                ⁢                                                  C                          ⁢                                                                                    Δ                              ⁢                                                                                                                          ⁢                                                              V                                CM                                                                                                                    t                              r                                                                                                                                                                                                              ⁢                                                      [                  Formula          ⁢                                          ⁢          1                ]            D: electric flux densityt: timen: unit normal vectorS: conductor areaρ: charge density on input sideQ: value of integral of charge density ρC: capacitance between input and outputV: voltage between input and outputVCM: common mode voltagetr: rising time
Since the through rate of the common mode voltage VCM is “ΔVCm/tr”, if the through rate is high, a large displacement current id flows from the input side (light emitting device side) to the output side (light receiving device side). A malfunction not occurring even if a large displacement current flows (possible to maintain an output state of the photocoupler as a correct state) means that the displacement current that is flowing does not easily enter the light receiving device.
As one method of improving the CMR characteristic, Patent Document 1 for example, discloses that a conductive polysilicon layer with translucency is formed in a light receiving part, as in the light receiving IC, and the potential of the polysilicon layer is grounded. With this method, an output malfunction is prevented by the displacement current being received by the polysilicon layer and being released to ground.
[Patent Document 1]
JP Patent Kokai Publication No. JP04-354379A
[Patent Document 2]
JP Patent Kokai Publication No. JP07-122779A
[Non-Patent Document 1]
Foundations and Applications of Optical Devices: Kiyoshi Ebina, (author and editor), (in Japanese) CQ Publishing, June 2005, pp. 253 and 270.